The consumption of plastic pipes such as polyethylene pipes for gas and water supply piping is gradually increasing. Of these plastic pipes, ones with a diameter of from 8 to 50 mm are shipped in the form of a coil of a prescribed length. For this purpose it is the usual practice to measure the length of a plastic pipe by means of a measuring roll while winding the plastic pipe into a coil and cut the plastic pipe into a prescribed length in the final stage of the manufacturing process.
In the above-mentioned measurement of length of a plastic pipe by means of a measuring roll, the measured value of length varies depending upon changes in the pressure of the measuring roll applied onto the plastic pipe surface, changes in the travelling speed of the plastic pipe, deformation of the plastic pipe under the pressure of the measuring roll, or the difference in the surface condition of the plastic pipe, and this causes the decrease in the measuring accuracy. Furthermore, since the length of a plastic pipe varies under the effect of the temperature, improvement of the measuring accuracy requires measurement indoors at a certain temperature of 23.degree..+-.2.degree. C., for example, but this is very difficult in terms of equipment.
To ensure a prescribed length of the shipped plastic pipe, therefore, it is a common practice to wind a plastic pipe into a coil slightly longer than the prescribed length as well as to sample a coil at random to actually measure the length of the uncoiled plastic pipe by means of a measuring tape, for example. It is not however easy to uncoil a coiled plastic pipe and measure the length thereof by means of a measuring tape or the like, requiring much time.
There is also known another method for determining a length of a plastic pipe, which comprises: measuring a total weight and dimensions of the cross-sectional area of a coiled plastic pipe, and determining a length of the plastic pipe on the basis of the total weight and the dimensions of the cross-sectional area thus measured of the plastic pipe and the density of the material of the plastic pipe. However, the above-described method, not being an actual measurement of the length of the plastic pipe, has only a low accuracy and permits only rough inspection of the length of the coiled plastic pipe as a sampling check.
Under such circumstances, there is a demand for the development of a method and an apparatus for determining a length of a pipe utilizing sound waves, which permit easy measurement at a high accuracy of the length of any of various pipes such as a plastic pipe irrespective of whether the pipe is straight or wound into a coil, but a method and an apparatus having such properties have not as yet been proposed.